For biological, medical, chemical research and applications, miniature microfluidic devices are required to form lab-on-a-chip units. In addition, it is desirable to integrate devices for micro-optics, micro-mechanics or even microelectronics to achieve complete lab-on-a-chip units. Most of these devices are manufactured using glass for achieving long term stability, chemical resistance and inertness. In addition, optical waveguides embedded in a transparent substrate is required to form integrated optic circuits and module.
Photo-Definable Glass
Glass fabrication technologies for both bulk and surface micromachining include isotropic chemical etching, laser micromachining, mechanical sawing, powder blasting, thermal moulding and photo-structuring. As the wet etching of glass is isotropic and the rates for anisotropic dry etching are often slow, a class of photosensitive glass was developed. The main constituent components of the photosensitive glass are: SiO2 65-85%, Li2O 7-19%, K2O 2-6%, Al2O3 3-9%, Na2O 1-3%, ZnO 0-2%, Sb2O3 0.03-0.4%, Ag2O 0.05-0.15%, CeO2 0.01-0.05% [T. R. Dietrich, W. Ehrfeld, M. Lacher, M. Krämer and B. Speit, Fabrication techniques for microsystems utilizing photoetchable glass, Microelectronic Engineering, (1996) vol. 30, pp. 497-504]. The conventional photo-structuring of these photosensitive glasses include the processes of: (1) an exposure under UV light through a mask to define the desired patterns, (2) a two-step annealing, first at 500° C. then at 600° C. each for 1 hour, allowing for the diffusion of reduced Ag atoms in the UV exposed regions to form nuclei and crystallize into lithium-metasilicate, Li2SiO3, and (3) wet etching in diluted 5-10% HF solutions to remove selectively the crystallized glass ceramic in the UV exposed regions. Due to the large etching selectivity of about 30 between the UV exposed regions and the unexposed ones, micro patterns with a depth up to several thousand micrometers can be created in these photo-definable glasses.
The reactions and mechanisms for each process step can be described as follows according to T. R. Dietrich et al:
A) During the melting process, Ce+3 ions are formed and stabilized by the sensitizer Sb2O3:2Ce+4+Sb+3← →2Ce+3+Sb+5  (1)B) When being illuminated by UV light, Ce+3 ions absorb photons and returns to the more stable Ce+4 form:Ce+3+hν→Ce+4+e−1  (2)
The released electron is captured by an Ag ion and reduces it to an Ag atom:Ag+1+e−1→Ag  (3)C) When being heated at temperatures of about 500° C., the reduced Ag atoms in the UV exposed regions diffuse to form nuclei. When further heated at temperatures of about 600° C., the glass crystallizes around the Ag nuclei to form lithium-metasilicate: Li2SiO3 with crystallite dimensions in the range of 1 μm to 10 μm.D) When etched in a diluted HF solution (5%-10% HF), the glass ceramic in the UV exposed regions will be etched at a rate about 20 to 30 times faster than that in the unexposed regions:SiO2+4HF→SiF4+2H2O  (4)
The etching rate of the UV exposed regions can be as high as 10 μm/minute, allowing structures as deep as 500 μm and with an aspect ratio as large as 10 to be created in these photosensitive glasses.
Wavelengths of UV light sources used for the exposure should be selected to be less than 330 nm in order to obtain sufficient photon absorption for the Ag reduction [K. H. Tantawi, E. Waddel and J. D. Williams, Structural and composition analysis of Apex™ and Foturan™ photodefinable glasses, Journal of Materials Sciences, (2013) vol. 48, pp. 5316-53231.]. Due to the variation of transmission or absorption of light in the photosensitive glasses with wavelength, the optimal dose in J/cm2 will be dependent on both the wavelength and the required etch rates. In ref. [K. H. M. Tantawi, J. Oates, R. Kmmali-Sarvestani, N. Bergquist and J. D. Williams, Processing of photosensitive APEX™ glass structures with smooth and transparent sidewalls, Journal of Micromechanics and Microengineering, (2011) vol. 21, pp. 017001, 6 pp], the optimal doses at 280 nm wavelength for different etch depths have been reported for Apex™ to be from 0.048 J/cm2 to 9.6 J/cm2 for etch depths from 10 μm to 2,000 μM.
Using optical beam exposure to create microstructures in photodefinable glass, the dimensions and shape on planes perpendicular to the direction of optical beam (z direction) are determined by the dimensions and shape of optically exposed regions. However, the dimension parallel to the direction of the optical beam is more difficult to control due to the small absorption and long penetration of exposure light in the photodefinable glass. This is due to the small optical absorption coefficients and relatively large transmission in near UV regions. Hence, the variation in the amount of photons absorbed with the distance in z-direction in the illuminated region is gradual and does not have a sharp transition. Therefore, during the subsequent chemical etching, the depth of the etched cavities will increase with etching time. Therefore, precise etching rate data at different depth z and the etching time control will be required in order to achieve the desired depth of the cavities. In addition, it is very difficult in applications which require several cavities each with a different cavity depth.
In addition to the photostructuring using UV light sources, which creates patterns and structures on substrate surfaces, pulses lasers operating at different wavelengths from 355 nm to 800 nm have been employed for exposure to create 3D structures and structures on or inside the photosensitive glasses [M. Masuda, K. Sugioka, Y. Cheng, N. Aoki, M. Kawachi, K. Shihoyama, K. Toyoda, H. Helvajian, K. Midorikawa, Applied Physics A, (2003) vol 76 pp. 857. H. Helvajian, P. D. Fuqua, W. W. Hansen, S. Jason, RIKEN Review, (2001) No. 32, pp. 57-63]. Using lasers with wavelengths larger than 350 nm, the absorption for the photosensitive glasses is small. Photo exposure can be effective only near the focal point where the light intensity is sufficiently large to cause multi photon absorption for excitation of Ce+3:Ce+3+nhν→Ce+4+e  (5)
The generated electron e−1 then will cause reduction of Ag ions into Ag atoms. Outside the focal point, the intensity of light is small and is not sufficient to modify the glass to cause crystallization in subsequent annealing. For example, using a pulse laser at 800 nm wavelength, arrays of lenses with a width of about 100 μm and a height of 100 μm have been created [C. H. Lin, L. Jiang, Y. H. Chai, H. Xiao, S. J. Chen and H. L. Tsai, Fabrication of microlens arrays in photosensitive glass by femtosecond laser direct writing, Applied Physics A, (2009) vol 97, pp. 751-757].
Interactions Between Electrons and a Solid
When a beam of electrons accelerated to a given energy Eo is incident on a solid, a simplified semi-empirical theory [K. Kanaya and S. Okayama, Penetration and energy-loss theory of electrons in solid targets, Journal of Physics D, (1972) vol 5, pp. 43-58] may be adopted to describe the subsequent events. The semi-empirical theory is often used to understand the principles of electron probe microanalysis, scanning electron microscopy, and electron beam writing. When the electrons penetrate into the solid target, electrons may be scattered either elastically or inelastically. The stopping of electrons could be due to inelastic collisions with atomic electrons in which the incident electron excites or ejects atomic electrons with loss of energy. The stopping of electrons could also be due to nuclear interactions, arises from elastic collisions with atomic nuclei, with transfer of both energy and momentum.
Hence, the incident electrons will travel straight into the target, suffering energy losses due to the electronic collisions, and be deflected by the nuclear collisions. The activities of electrons incident on a solid can be described by a diffusion model and is graphically shown in FIG. 1(a).
An electron beam (120) with electrons accelerated to an energy value of Eo is allowed to be incident on the substrate top surface (105T) of a substrate (100), which has a substrate thickness (110) and a bottom surface (105B, see FIG. 1(a)). The incident electrons in the electron beam (120) will travel straight into the substrate and reach the point called electron diffusion center (130). Due to the interactions with electrons and nuclei, the activities of the incident electrons shown in the two dimensional drawing in FIG. 1(a) consist of segmented electron paths (136). The end of each segmented path away from the electron diffusion center (130) represents the maximum distance the electron can travel. Hence, the distribution of (penetrating) incident electrons can be described by the electron diffusion center (130), an electron diffusion radius (155), defining an electron diffusion sphere (140) and an electron diffusion depth xD (150). The maximum distance the electrons can travel in the direction of the electron beam (120) is called the electron penetration depth or electron range R (160). Within the electron diffusion sphere (140), incident electrons loss energy and may be absorbed by the materials of the substrate (100). It is noted that in FIG. 1(a), the electron diffusion sphere (140) is below the substrate top surface (105T) with a substrate top surface-to-electron diffusion sphere distance (170). The electron diffusion depth xD (150) and hence the electron penetration depth R (160) is determined by the accelerating voltage V or electron energy Eo and the atomic number (Z) of the substrate materials. Extensive studies have been made in the past decades on the electron range with accelerating voltage on different substrates having different atomic numbers. FIG. 1(b) shows variation of mass-range product, ρR, with energy Eo or accelerating voltage V for two materials: C with Z=6 and Al with Z=13. Here ρ is density of the substrate material. In the following description, diffusion center may be used to represent electron diffusion center, diffusion radius may be used to represent electron diffusion radius, diffusion sphere may be used to represent electron diffusion sphere, diffusion depth may be used to represent electron diffusion depth, whereas penetration depth or range may be used to represent electron penetration depth of electron range.
As the value of Eo is increased from 10 keV to 100 keV, the value of ρR increases by about 30 times. Therefore, the value of ρR is not directly proportional to Eo and is given by the following equation [K. Kanaya and S. Okayama, Penetration and energy-loss theory of electrons in solid targets, Journal of Physics D, (1972) vol 5, pp. 43-58.]:ρR=5.025×10−12A(g)Eo5/3/λsZ8/9  (6)
Here A(g) is the atomic weight of substrate material and λs is a constant determined empirically. For the electron definable glass to be disclosed in the invention, the materials are mainly SiO2, (Si: Z=14, O: Z=8) with small portions of other metal oxides. To simplify the description and consideration, average atomic number of photo-definable glass is taken as 10.
It is also noted that the ratio of electron diffusion depth xD (150) to electron penetration depth R (160): xD/R for an electron beam with a given electron energy Eo is not constant, but varies with the atomic number of the substrate materials. For an electron beam incident on a substrate material so that xD/R=0.5, the electron diffusion depth (150) is equal to the electron diffusion radius (155) and the electron diffusion sphere (140) will get in touch with the substrate top surface (105T). When xD/R<0.5, the upper part of the electron diffusion sphere (140) will emerge from the substrate top surface (105T). Under this condition, the substrate materials within the electron diffusion sphere will not form a complete sphere. Conversely, when xD/R>0.5, the entire electron diffusion sphere (140) created will be below the substrate top surface (105T). Under this condition, the substrate materials within the electron diffusion sphere will form a complete sphere and there is a finite distance between the substrate top surface (105T) and the electron diffusion sphere (140). The variation of xD/R in terms of Z, based on a model for 12/(Z+8) [K. Kanaya and S. Okayama, Penetration and energy-loss theory of electrons in solid targets, Journal of Physics D, (1972) vol 5, pp. 43-58] is shown in FIG. 1(c). It shows a continuous decrease of xD/R with the increase in Z. For substrate materials with Z values less than 18, xD/R is greater than 0.5 whereas for substrate materials with Z values greater than 18, xD/R is less than 0.5. For a photodefinable glass with the main contents of SiO2, the average atomic number is 10 (=(14+8+8)/3). Therefore, the xD/R value for SiO2 can be taken as 0.7 from FIG. 1(c).
Optical Waveguides
The demands for ever faster and higher data transfer in optical communications have stimulated development and research on integrated optics and optical circuits capable of more complex functions. The enhanced development and research have resulted in various miniature optical components such as optical switches, couplers, waveguides and filters on a planar substrate. To create the integrated devices, it is necessary to create optical waveguides in or on the substrates. Optical waveguides are often created by impurity diffusion or ion exchange, deposition and etching. However, the dimensional requirements have made conventional fabrication to be expensive for optical communications. FIG. 2 shows a schematic cross-sectional diagram of an optical waveguide or optical fibre (200). Here, (210) is a core with a core index of ncore and a core diameter dcore (220), a cladding (230) with a cladding thickness (240) and a cladding refractive index ncladding. In order to allow the incident beam (250) which is incident at an incident angle θ (260) with respective to the waveguide axis (270) to enter and propagate in the core (210), the incident angle θ (260) must satisfy the conditions:sin θ<NA=[n2core−n2cladding]1/2  (7)
Here, NA is the numerical aperture. After propagation, output beam (280) will exit the core of the fibre. Assuming a refractive index value of 1.50 for the core (210), the values of NA and acceptance angle θ or the maximum incident angle to allow incident beam to be coupled into the waveguide for different cladding indices are shown in Table 1. It is noted that the acceptance angle increases with the decrease in the cladding index.
TABLE 1Relationship between relative refractive index differenceand numerical aperture for core refractive index = 1.50RelativeCladdingAcceptanceRef. indexindexNAangle θc0.20%1.4970.10 6°0.40%1.4940.13 8°0.80%1.4880.1911°1.00%1.4850.2112°1.47%1.4780.2615°
It is thus clear that in order to create optical waveguides the refractive index for the core should be greater than the refractive index of the cladding so that optical beams can be confined within the core for propagation.
Core Diameter of Optical Waveguides
It is further noted that in order to support the desired modes for propagation, it is required to control the core diameter dcore (220) of the optical waveguide or fibre. A typical dimension or diameter is in the range of 6-10 μm, whereas the thickness of cladding (240) is preferably 0.5 μm or more. Therefore, extensive diffusion for different methods will be required in creating the waveguide or fibre.
It has been reported that optical waveguides can be created within a photosensitive glass by femtosecond lasers [Z. L. Li, D. K. Low, M. K. Ho, G. C. Lim and K. J. Moh, Journal of Laser Applications, (2006) vol. 18, pp. 320-324]. The waveguides are created by shinning light in the photosensitive glass which causes an increase in the refractive index in the illuminated regions. In the areas subjected to optical illumination and/or heat treatment, the refractive index in the illuminated areas is increased as compared to the surrounding areas of the photosensitive glass not subjected to the optical illumination. However, the aspect ratio of the created waveguides is quite large (2 to 6.5).
When using optical beam exposure to create microstructures in photodefinable glass, the dimensions and shape on planes perpendicular to the direction of optical beam (z direction) are determined by the dimensions and shape of optically exposed regions. However, the dimension parallel to the direction of the optical beam is more difficult to control due to the small absorption of exposure light by the photodefinable glasses. Due to the small optical absorption coefficients, the optical dose in the illuminated region in the z-direction does not have sharp transition and the variation of optical dose, i.e. the amount of light absorbed per unit distance in z-direction varies only gradually. As a result, the depth of the etched cavities will increase with etching time during the subsequent chemical etching. Therefore, precise etching rate data at different depth z and the etching time control will be required in order to achieve the desired depth of the cavities. It is very difficult in applications to obtain several cavities each with a different cavity depth.
The present invention teaches methods for the creation of microstructures and optical waveguides in definable glass by exposure using electron beams.